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LSF0204 pull-up resistor

Emkay Kim
Prodigy 270 points

Hi,

could you please clarify where I (must) need to put pull-up resistor??

 only case 4 is bi-directional.

  1. MCU UART Tx(Push pull) --> LSF0204RUTR A side --> LSF0204RUTR B side --> Peripheral UART Rx
  2. MCU UART Rx <-- LSF0204RUTR A side <-- LSF0204RUTR B side <-- Peripheral UART Tx 
  3. MCU GPIO Tx(Push pull) --> LSF0204RUTR A side --> LSF0204RUTR B side --> Peripheral GPIO Rx
  4. MCU I2C Data <--> LSF0204RUTR A side <--> LSF0204RUTR B side <--> Peripheral I2C Data
  5. MCU I2C Clock --> LSF0204RUTR A side --> LSF0204RUTR B side --> Peripheral I2C Clock

 please let me know each side, A,B whether pullup resistor is needed or not.

and if there's no need to use pullup resistor, then should I just tie the pin with Vcc directly? or GND? or Open?

Thanks!

  • 0
    TI__Guru* 97046 points

    Hi Emkay,

    Please take a look at the "Frequently Asked Questions" section of this application report.  There is a very good summary there of exactly when and where pull-up resistors are required with the LSF family of level translators.

  • 0 in reply to Emrys Maier
    TI__Prodigy 270 points
    Hi Maier,

    I've already looked into it, and talked with apps ppl before.
    I'm trying to get confirmation on this, whether I understood correctly or not.
    please give me answers of my questions, and it will help me a lot.
    this is quite urgent issue.

    Thanks.
  • 0 in reply to Emkay Kim
    TI__Guru* 97046 points
    I will need voltage levels for each side of the LSF, as well as current drive to/from each device (including input leakage currents) to recommend pull-up sizes.
  • 0 in reply to Emrys Maier
    TI__Prodigy 270 points
    Thanks Maier,

    A side : 1.8V
    B side : 3.3V
    current drive : will talk to customer and recommend 300ohm to 1kohm , not to let the current over than 15mA.

    Thanks!
  • 0
    TI__Guru* 97046 points

    "if there's no need to use pullup resistor, then should I just tie the pin with Vcc directly? or GND? or Open?"

    None of those -- if a pullup is not required, the device is connected directly to the input or output.  That is all.  Once I have a better idea of your system, I will make a schematic to help you visualize the system.

  • 0 in reply to Emkay Kim
    TI__Guru* 97046 points

    Emkay,

    I have built a schematic to show where pull-ups are required.  There is a ? on the UART return line's high-side pullup.  Since I do not know the output type of the Peripheral UART Tx, I cannot make a recommendation.

    ? - If the device is push-pull, a high-side pull-up is not required.  If the device is open-drain or open-collector, a pull-up is required.

     

    The simple rules to follow:

    1. An open drain connection will ALWAYS require a pull-up and a push-pull will never require a pull-up.

    2. A passive receive on the LOW side will not require a pull-up (unless it has significant leakage current) and a passive receive ont he HIGH side will always require a pull-up.

  • 0 in reply to Emrys Maier
    TI__Prodigy 270 points
    Maier,

    Thanks for your sincere reply, it helped me to confirm the application note.

    on the A2 side, it's Low side and recevier, so that there's no pull up needed, right?
    in the application note, if there's signal integrity(noise problem), then pull up can cover it, but it's not mendatory. correct?
  • 0 in reply to Emkay Kim
    TI__Guru* 97046 points
    That's right -- I would keep it just to be safe.